To cope with the increasing demand for wireless data traffic after commercialization of 4G communication systems, active efforts are underway to develop enhanced 5G or pre-5G communication systems. As such, 5G or pre-5G communication systems are referred to as beyond 4G communication systems or post LTE systems.
To achieve high data rates, use of the extremely high frequency (mmWave) band (e.g. 60 GHz band) is expected in a 5G communication system. To reduce propagation pathloss and to increase propagation distance at the mmWave band, use of various technologies such as beamforming, massive MIMO, full dimensional MIMO (FD-MIMO), array antenna, analog beamforming and large scale antenna are under discussion for 5G communication systems.
To enhance system networks, various technologies such as evolved or advanced small cell, cloud radio access network (cloud RAN), ultra-dense network, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP) and interference cancellation are under development for 5G communication systems.
In addition, for 5G communication systems, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) are under development for advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA) and sparse code multiple access (SCMA) are under development for advanced access.
In the LTE system as a representative example of a wideband wireless communication system, Orthogonal Frequency Division Multiplexing (OFDM) is used for the downlink (DL) and Single Carrier Frequency Division Multiple Access (SC-FDMA) is used for the uplink (UL). The uplink refers to a radio link through which a user equipment (UE) or mobile station (MS) sends a data or control signal to a base station (BS or eNode B), and the downlink refers to a radio link through which a base station sends a data or control signal to a user equipment. In such multiple access schemes, time-frequency resources used to carry user data or control information are allocated so as not to overlap each other (i.e. maintain orthogonality) to thereby identify the data or control information of a specific user.
The LTE system employs Hybrid Automatic Repeat reQuest (HARQ) to retransmit data at the physical layer when a decoding error has occurred in the initial transmission. HARQ is a scheme that enables the receiver having failed in decoding data to transmit information (NACK) indicating the decoding failure to the transmitter so that the transmitter can retransmit the corresponding data at the physical layer. The receiver may combine the retransmitted data with the previously received data for which decoding has failed, increasing data reception performance. When the data is correctly decoded, the receiver may send information (ACK) indicating successful decoding to the transmitter so that the transmitter can transmit new data.
FIG. 1 illustrates a basic structure of the time-frequency domain in the downlink of the LTE system serving as radio resources for transmitting data or control channels.
In FIG. 1, the horizontal axis denotes the time domain and the vertical axis denotes the frequency domain. In the time domain, the minimum unit for transmission is OFDMA symbols, Nsymb OFDMA symbols 102 constitute one slot 106, and two slots constitute one subframe 105. The length of a slot is 0.5 ms and the length of a subframe is 1.0 ms. The radio frame 114 is a time domain unit composed of 10 subframes. In the frequency domain, the minimum unit for transmission is subcarriers, and the total system bandwidth is composed of total NBW subcarriers 104.
The basic unit of resources in the time-frequency domain is a resource element (RE) 112. The RE may be represented by an OFDM symbol index and a subcarrier index. A resource block (RB, or physical resource block (PRB)) 108 is defined by Nsymb consecutive OFDM symbols 102 in the time domain and NRB consecutive subcarriers 110 in the frequency domain. Hence, one RB 108 is composed of Nsymb×NRB REs 112. In general, the minimum unit for data transmission is a resource block. Normally, in the LTE system, Nsymb is set to 7 and NRB is set to 12, and NBW and NRB are proportional to the system transmission bandwidth. The data rate may increase in proportion to the number of resource blocks scheduled for the user equipment. The LTE system defines and operates six transmission bandwidths. In the case of an FDD system where downlink and uplink frequencies are separately used, the downlink transmission bandwidth may differ from the uplink transmission bandwidth. The channel bandwidth denotes an RF bandwidth corresponding to the system transmission bandwidth. Table 1 illustrates a correspondence between the system transmission bandwidth and the channel bandwidth defined in the LTE system. For example, the transmission bandwidth of an LTE system having a channel bandwidth of 10 MHz is composed of 50 resource blocks.
TABLE 1Channel bandwidth BWchannel[MHz]1.435101520Transmission615255075100bandwidthconfiguration NRB
In a subframe, N initial OFDM symbols are used to transmit downlink control information. In general, N={1, 2, 3}. The value of N varies for each subframe according to the amount of control information to be sent at the current subframe. The control information may include a control channel transmission interval indicator indicating the number of OFDM symbols carrying control information, scheduling information for downlink data or uplink data, and HARQ ACK/NACK signals.
In the LTE system, scheduling information for downlink data or uplink data is sent by the base station to the UE in the form of Downlink Control Information (DCI). Various DCI formats are defined. The DCI format to be used may be determined according to various parameters related to scheduling information for uplink data (UL grant), scheduling information for downlink data (DL grant), compact DCI with a small size, spatial multiplexing using multiple antennas, and power control DCI. For example, DCI format 1 for scheduling information of downlink data (DL grant) is configured to include at least the following pieces of control information.                Resource allocation type 0/1 flag: this indicates whether the resource allocation scheme is of type 0 or type 1. Type 0 indicates resource allocation in the unit of Resource Block Group (RBG) by use of a bitmap. In the LTE system, the basic scheduling unit is a resource block (RB) represented as a time-frequency domain resource. An RBG including multiple RBs is the basic scheduling unit for type 0. Type 1 indicates allocation of a specific RB in one RBG.        Resource block assignment: this indicates an RB allocated for data transmission. The resource represented by resource block assignment is determined according to the system bandwidth and resource allocation scheme.        Modulation and coding scheme (MCS): this indicates the modulation scheme applied for data transmission and the transport block size for data to be sent.        HARQ process number: this indicates the process number of the corresponding HARQ process.        New data indicator: this indicates either initial transmission for HARQ or retransmission.        Redundancy version: this indicates the redundancy version for HARQ.        TPC (Transmit Power Control) command for PUCCH: this indicates a TPC command for Physical Uplink Control Channel (PUCCH) being an uplink control channel.        
DCI is channel coded, modulated, and sent through Physical Downlink Control Channel (PDCCH or control information) or EPDCCH (enhanced PDCCH or enhanced control information).
In general, for each UE, DCI is scrambled with a specific Radio Network Temporary Identifier (RNTI, or UE ID), appended by a cyclic redundancy check (CRC) value, channel coded, and transmitted via independent PDCCH. In the time domain, PDCCH is mapped and transmitted during the control channel transmission interval. In the frequency domain, the mapping position of PDCCH is determined by the identifier (ID) of each UE and PDCCH is dispersed across the overall system transmission bandwidth.
Downlink data is sent via Physical Downlink Shared Channel (PDSCH) serving as a physical downlink data channel. The PDSCH is sent after the control channel transmission interval. Scheduling information for PDSCH such as mapping positions in the frequency domain or the modulation scheme is notified by DCI transmitted on the PDCCH.
The base station uses the 5-bit MCS field of control information constituting DCI to notify the UE of the modulation scheme applied to PDSCH (to be sent to UE) and the size of data to be sent (transport block size (TBS)). TBS indicates the size of a transport block (TB) before channel coding is applied for error correction.
Modulation schemes supported by the LTE system include QPSK (Quadrature Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation), and 64QAM, whose modulation order (Qm) is 2, 4 and 6, respectively. That is, it is possible to send 2, 4, and 6 bits per symbol by using QPSK, 16QAM, and 64QAM, respectively.
FIG. 2 is an illustration of a time-frequency domain structure of PUCCH transmission in the LTE-A system according to a related art. In other words, FIG. 2 illustrates a time-frequency domain structure of PUCCH transmission in the LTE-A system where PUCCH (Physical Uplink Control Channel) is a physical layer control channel through which the UE sends Uplink Control Information (UCI) to the base station.
The UCI may include at least one of the following pieces of control information.                HARQ-ACK: when no error is found in downlink data received from the base station through Physical Downlink Shared Channel (PDSCH, serving as a downlink data channel) to which HARQ is applied, the UE feedbacks ACK (Acknowledgement); and when an error is found therein, the UE feedbacks NACK (Negative Acknowledgement).        Channel Status Information (CSI): this includes Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI), Rank Indicator (RI), and signal carrying downlink channel coefficients. To achieve a desired level of data reception performance, the BS may set the Modulation and Coding Scheme (MCS) for data to be sent to the UE to a suitable value on the basis of CSI information obtained from the UE. The CQI indicates the signal to interference and noise ratio (SINR) for the full system bandwidth (wideband) or a part thereof (subband) and is normally represented as an MCS value indicating a specific level of data reception performance. The PMI/RI indicates precoding and rank information needed by the BS to send data through multiple antennas in a system supporting Multiple Input Multiple Output (MIMO). The signal carrying downlink channel coefficients may provide more detailed channel status information compared with the CSI signal, but with increased uplink overhead. Here, the UE is notified in advance, through higher layer signaling, by the BS of CSI configuration information, such as reporting mode indicating specific information items to be fed back, resource information indicating resources to be used, and transmission period. The UE sends CSI information to the BS by use of the CSI configuration information received in advance.        
In FIG. 2, the horizontal axis denotes the time domain and the vertical axis denotes the frequency domain. In the time domain, the minimum unit for transmission is SC-FDMA symbols 201, NsymbUL SC-FDMA symbols constitute one slot 203 or 205, and two slots constitute one subframe 207. In the frequency domain, the minimum unit for transmission is subcarriers, and the total system transmission bandwidth 209 is composed of total NBW subcarriers. The value of NBW is proportional to the system transmission bandwidth.
The basic unit of resources in the time-frequency domain is a resource element (RE).
The RE may be represented by an SC-FDMA symbol index and a subcarrier index. A resource block (RB) 211 or 217 is defined by NsymbUL consecutive SC-FDMA symbols in the time domain and NscRB consecutive subcarriers in the frequency domain. Hence, one RB is composed of NsymbUL×NscRB REs. In general, the minimum unit for transmitting data or control information is a resource block. The PUCCH is mapped to one RB in the frequency domain and transmitted for one subframe.
FIG. 2 illustrates a case where NsymbUL=7, NscRB=12, and NRSPUCCH=2 (the number of reference signals in one slot for channel estimation). Reference signals (RS) use constant amplitude zero auto-correlation (CAZAC) sequences. CAZAC sequences have a constant amplitude and have an autocorrelation of zero. When a given CAZAC sequence is cyclically shifted (CS) by a value greater than the delay spread of the propagation path to produce a new CAZAC sequence, the original CAZAC sequence and the new CAZAC sequence are orthogonal. Hence, a CAZAC sequence of length L may be used to generate up to L cyclically-shifted orthogonal CAZAC sequences. The length of a CAZAC sequence applied to the PUCCH is 12 (the number of subcarriers constituting one RB).
The UCI is mapped to a SC-FDMA symbol to which an RS is not mapped. FIG. 2 shows a case where total 10 UCI modulation symbols d(0), d(1), . . . , d(9) (213 and 215) are mapped respectively to SC-FDMA symbols in one subframe. To multiplex UCI information of different UEs, each UCI modulation symbol is multiplied by a CAZAC sequence cyclically-shifted by a given value and mapped to the corresponding SC-FDMA symbol. To achieve frequency diversity, frequency hopping is applied to the PUCCH on a slot basis. The PUCCH is placed at an outer portion of the system transmission bandwidth so that the remaining portion thereof may be used for data transmission. For example, in the first slot of a subframe, the PUCCH is mapped to RB 211 disposed at an outermost portion of the system transmission bandwidth. In the second slot, the PUCCH is mapped to RB 217 disposed at another outermost portion of the system transmission bandwidth, where the frequency for RB 217 is different from that for RB 211. In general, the positions of the RBs to which the PUCCH for sending HARQ-ACK information and the PUCCH for sending CSI information are mapped do not overlap each other.
In the LTE system, for the PDSCH (physical layer channel for downlink data transmission) or the PDCCH/EPDDCH containing semi-persistent scheduling (SPS) release, the timing of the PUCCH or PUSCH (uplink physical layer channel sending HARQ ACK/NACK) may be fixed. For example, in the LTE system operating in frequency division duplex (FDD) mode, for the PDSCH or PDCCH/EPDCCH containing SPS release transmitted at n−4th subframe, HARQ ACK/NACK is sent through the PUCCH or PUSCH at nth subframe.
The LTE system adopts an asynchronous HARQ scheme where the data retransmission timing is not fixed in the downlink. That is, when HARQ NACK is fed back by the UE in response to initial data transmission from the BS, the BS may determine the retransmission timing freely according to the scheduling operation. For HARQ operation, the UE buffers the data causing a decoding error and combines the buffered data with the next retransmission data.
The LTE system adopts a synchronous HARQ scheme having fixed data transmission points in the uplink unlike downlink HARQ. That is, the uplink/downlink timing relationship among Physical Uplink Shared Channel (PUSCH), Physical Downlink Control Channel (PDCCH) followed by the PUSCH, and Physical Hybrid Indicator Channel (PHICH) carrying downlink HARQ ACK/NACK corresponding to the PUSCH are fixed according to the following rules.
If the PDCCH carrying uplink scheduling control information or the PHICH carrying downlink HARQ ACK/NACK is received from the BS at nth subframe, the UE transmits the PUSCH carrying uplink data corresponding to the control information at n+kth subframe. Here, k is specified differently for the FDD or TDD (time division duplex) mode and its configurations.
For example, k is fixed to 4 for the FDD LTE system.
If the PHICH carrying downlink HARQ ACK/NACK is received from the BS at ith subframe, the PHICH corresponds to the PUSCH having been transmitted by the UE at i-kth subframe. Here, k is specified differently for the FDD or TDD mode and its configurations.
For example, k is fixed to 4 for the FDD LTE system.
For a cellular wireless communication system, one of important performance criteria is the latency of packet data. To this end, in the LTE system, signals are sent and received on a subframe basis with a transmission time interval (TTI) of 1 ms. The LTE system may support UEs with a shortened-TTI less than 1 ms (shortened-TTI UE or shorter-TTI UE). Shortened-TTI UEs can be suitable for latency-critical services such as Voice over LTE (VoLTE) and remote control services. Shortened-TTI UEs can also be used to realize cellular-based mission critical Internet of Things (IoT).
In the current LTE or LTE-A system, base stations and user equipments are designed to transmit and receive on a subframe basis with a 1 ms TTI. To support a shortened-TTI UE with a TTI less than 1 ms in an environment where regular BSs and UEs operate with a 1 ms TTI, it is necessary to specify transmission and reception operations different from those of a regular LTE or LTE-A UE. Accordingly, the present invention proposes a detailed scheme that enables a regular LTE or LTE-A UE and a shortened-TTI UE to operate together in the same system.